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The tale of a very shy comet...
Jan 30, 2012
For only the second time in over 2,200 comet discoveries, the SOHO satellite has a new short-period comet discovery to add to its name, and it has the STEREO-B satellite to thank for it! This is a very cool discovery, and for me incredibly satisfying as it answers a question that has been bugging me since I inherited the Sungrazer Project back in late 2003. But it is also a story of a comet that seems almost to have a personality of its own... and not a very friendly one!
The first time we saw this comet was back in 2003, in the SOHO/LASCO C3 images. It was so faint, we almost missed it! (click for larger image) So we begin -- way back in 2003 -- when amateur astronomer and SOHO comet hunter Jim Danaher spotted a small, faint comet arcing through the SOHO/LASCO C3 images. This comet had no tail, no obvious fuzzy coma, and for the most part just faded in and out of the visibility as its brightness only bordered on the limit of detectability in the LASCO camera (typically about 8th magnitude). Unlike the Sungrazers that we see every few days that fly towards the Sun in the center of the images, this one was seemingly shy, moving almost horizontally from left-to-right near the lower part of the LASCO images like it was trying to sneak past without anyone noticing it. Alas, no such luck for this shy iceball as it was spotted, reported, and puzzled over by the ever-vigilant SOHO comet hunters.
At the time, the Sungrazer project was actually headless; I hadn't started working here, and my predecessor had already left. So unfortunately it was not until early in the following year that I got to look at, record, and measure this comet, send my results to the late Dr. Brian Marsden at the Minor Planet Center (Cambridge, MA). I was able to follow the comet in the LASCO C3 data for almost 24hrs, and recorded 71 unique observations of the comet, which I had unofficially named SOHO-673. Normally this many observations can result in a half-decent orbit, but this one had Brian scratching his head. Several solutions were possible but none of them made sense. A parabolic orbit would fit, but that put the comet in dark skies farther from the Sun. If that was the case, why was it not seen from the ground? A low-inclination, short period orbit was also possible, but that meant it must have passed undetected through the SOHO data before, and still afforded opportunities for ground based observers. This latter paradox could be fixed somewhat it the comet's aphelion (furthest point from the Sun) was placed inside Earth's orbit, but then the LASCO observations I had recorded no longer worked well. Was it perhaps an Appollo asteroid? Brian's closing remark to me was "I could declare the object as a comet and go with the aforementioned parabola, but people are going to ask questions...". We were really scratching our heads. Nonetheless, Brian did eventually go ahead with a parabolic orbit, and the assumed comet received the official designation of C/2003 T12 (SOHO), noting for the record the possibility "that C/2003 T12 has a short period and somewhat smaller perihelion distance".
People didn't actually ask too many questions, and thoughts of this mysterious comet were pushed from our mind as SOHO's unrelenting stream of new discoveries kept us well-occupied in the coming months. Nonetheless, several times over the following years I would find myself readdressing the comet, hoping someone would recover it, or that it would reappear. I really was beginning to lose hope that the mystery would be solved.
Fast forward to present day, and on January 19th 2012 I received a report from expert STEREO comet hunter Alan Watson (Australia) that he had noticed a "fuzzy object moving away from the Sun" in the STEREO/SECCHI HI-1B images. Typically these reports turn out to be a known, ground-discovered comet, so I held off on any action for a day or two to see how things shaped up. But then I saw a follow-up report that the comet had actually also been visible -- bright and unnoticed -- for several days prior in the COR-2 images on the same spacecraft. How sneaky! This got my attention and I set about getting measurements of this comet recorded, as it was now apparent that it was a new discovery and could potentially be ground-observable in the coming weeks. In the meantime, the Comet Hunters were doing their own calculations, and on January 23rd German amateur astronomer, and all-time leading SOHO comet discoverer, Rainer Kracht announced that he had been able to convincingly link observations of the new STEREO comet with C/2003 T12. This was definitely an "oh, wow" moment for me, and one of tremendous resolution and satisfaction (like the feeling of an itchy foot when you're driving, and you finally get to a light and can safely put the brake on and scratch it... it was much like that, except my foot had been itching for eight years!).
The movie on the left shows the comet in the STEREO/SECCHI COR2 images. It actually passed right through this instrument camera for several days without anyone seeing it. Like I said, sneaky...
So with my observations submitted, I was delighted when yesterday (Jan 29, 2012), comet P/2003 T12 (SOHO) was officially announced. The ~4.1yr periodic orbit orbit gives it a perihelion distance (closest point to the Sun) of just over 0.5 Astronomical Units (AU), or about half the distance from the Earth to the Sun, and aphelion (furthest from Sun) of just over 4.5AU, which isn't that far from Jupiter's orbit. In January 2008, P/2003 T12 (SOHO) came relatively close to Earth at just 0.176AU . That's certainly not nearly close enough for us to be concerned about, nor will this object ever be a threat to Earth or any other planet, but again makes me wonder why it wasn't seen from Earth.
So what is my scientific conclusion? This comet is shy, reclusive, and sneaky! Somehow it has existed in our inner solar system for millennia, returning every ~4yrs, occasionally buzzing relatively close to Earth, and yet has never been discovered. It tried to sneak through the SOHO camera's in 2003, managed to miss all ten SOHO and STEREO cameras in 2007/2008, and almost got overlooked in the STEREO-B cameras in 2012. And an ever-growing army of space and Earth-based sky-watchers have consistently missed it time and time again. But no longer is that so! We now know where it is at any given time and, as I write this, is see the first report of a probable ground-based observation of this comet from New Mexico. It is very faint already -- close to mag 15 -- but for a comet clearly so desperate to remain low-key, it doesn't surprise me in the least that it would become so faint so soon after passing the Sun (where it had no choice but to brighten up by several orders of magnitude, at least for a few days).
Now, P/2003 T12 will fly off to the recesses of the solar system, spending most of its time -- quite happily, I'm sure -- in that large antisocial void between Mars and Jupiter's orbits. However, much to the chagrin of this comet, the inevitable pull of the Sun will take its toll, and this comet will return once more in early 2016... no doubt as quietly and subtly as it possibly can.
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